1. Field of the Invention
The present invention relates to an image display apparatus and a method for fabricating the same; specifically, to an image display apparatus utilizing light emission from a rare gas discharge, which is used for a color television image receptor, a display and the like, and a method for fabricating the same. In particular, the present invention relates to a discharge electrode used in such an image display apparatus and a method for fabrication the same.
2. Description of the Related Art
Gas discharge type image display apparatuses such as a plasma display panel (hereinafter, referred to as "PDP") have been utilized as plane type image display apparatuses in information terminal equipment such as a computer. Since the PDPs are advantageous in clear image display and a wide viewing angle as compared with a liquid crystal panel, their application is extended.
As the television image receptor is made larger in size, projection type televisions using a Braun tube and a liquid crystal panel are more and more commercialized. However, such conventional projection type televisions have problems in luminance of the picture and size of the device.
On the other hand, the PDP has drawn attention as an image display device which can be remarkably thinned. Moreover, a technique for obtaining multi-color images in the PDP has been remarkably improved. As a result, the PDP attracts attention as a frontier of the image display device to realize a direct-view type wall-television with high definition. Such a condition requires precise reproducibility and for the life of the PDP to improve.
FIG. 20 is a perspective view showing the configuration of a typical DC type PDP 500.
The DC type PDP 500 includes: a front glass substrate 39 and a rear glass substrate 40 which are made of transparent glass and the like; and a plurality of discharge cells 41 constituted therebetween. A fluorescent material 42 emitting a light beam of a predetermined color is provided inside each of the discharge cells 41. A gas discharge occurs inside each of the discharge cells 41 so as to generate an ultra-violet ray, and the thus generated ultra-violet ray is radiated onto the fluorescent material 42, thereby performing a color display.
Specifically, a plurality of cathode lines 43 are formed on the surface of the front glass substrate 39, which faces the rear glass substrate 40, so as to be parallel to each other. A plurality of anode lines 44 are formed on the surface of the rear glass substrate 40, which faces the front glass substrate 39, so as to be parallel to each other and perpendicularly cross the cathode lines 43. Each of the cross points of the cathode lines 43 and the anode lines 44 corresponds to a single discharge cell 41. Each of the discharge cells 41 is separated from another discharge cell 41 by a partition wall 45 and forms a fine discharge tube. The fluorescent materials 42 respectively corresponding to red (R), green (G) and blue (B) are applied onto the respective discharge cells 41 in an appropriate arrangement. The partition wall 45 keeps the distance between the front glass substrate 39 and the rear glass substrate 40 at a predetermined value and prevents the colors of the adjacent discharge cells 41 from being mixed.
An insulating layer 46 is formed on the rear glass substrate 40. The insulating layer 46 is formed so as to expose the anode lines 44 at positions corresponding to the respective discharge cells 41 and to cover the anode rays 44 in the other region. A cell resistance (not shown in FIG. 20) for limiting the discharge current may be provided for each of the discharge cells 41.
A discharge gas for radiating an ultra-violet ray is sealed within each of the discharge cells 41. For example, a mixture of helium and xenon is sealed within the discharge cells 41 so that the gas pressure in the sealed cells 41 can reach about several hundreds Torr.
In the DC type PDP 500 having the above configuration, when a voltage is applied between an arbitrarily selected cathode line 43 and an arbitrarily selected anode line 44, a discharge occurs in the discharge cell 41 at the position corresponding to the cross point thereof. More specifically, electrons are emitted from the cathode lines 43 to reach the anode lines 44 while ionizing the discharge gas inside the discharge cells 41. The voltage applied for generating such a discharge is referred to as a writing voltage. The fluorescent materials 42 are excited by the ultra-violet rays generated by the ionization of the discharge gas which attends the discharge, whereby light beams in predetermined colors are emitted in each cell 41. In this way, a color display is performed.
FIG. 21 shows a method for applying a voltage pulse in the case where the DC type PDP 500 shown in FIG. 20 is driven by a refresh driving method.
The DC type PDP 500 includes cathode lines 43K1 to 43Kn, i.e., n cathode lines in total (collectively denoted by the reference numeral 43) and anode lines 44A1 to 44Am, i.e., m anode lines in total (collectively denoted by the reference numeral 44). Each of the cross points of the cathode lines 43 and the anode lines 44 corresponds to each of the discharge cells 41.
In the refresh driving method, a negative pulse voltage 48 is sequentially applied to the cathode lines 43K1 to 43Kn in a time-division manner so as to sequentially select the cathode lines 43. This operation is called scanning, and the cathode lines 43 may be called scanning lines.
Subsequently, the anode lines 44 corresponding to the discharge cells 41 which are expected to emit light beams are selected from the discharge cells 41 along the selected cathode lines 43 in a synchronous manner with the selection of any one of the cathode lines 43. This selection is performed by applying a positive pulse voltage 51 to the anode lines 44 to be selected. Therefore, if all anode lines 44 are simultaneously selected, all discharge cells 41 on one of the cathode lines 43 are simultaneously selected to emit light. By appropriately selecting the anode lines 44 in accordance with the information to be displayed by the selected cathodes lines 43, light can be emitted in an arbitrary pattern. In this way, an operation as an image display device is realized.
In the refresh driving method, light-emission occurs only when the writing voltage is applied, and an image is displayed by utilizing the thus emitted light. As the number of cathode lines 43 increases, the time period for a pulse application to each of the cathode lines 43 is shortened. Accordingly, the light-emission time in each of the cathode lines 43 is shortened in inverse proportion to the number of cathode lines 43. As a result, as the number of cathode lines 43 increases, the luminance of the image to be displayed is lowered.
A memory driving method is used to solve the above problems in the refresh driving method.
Generally, when the discharge occurs in the discharge cells 41 due to application of the writing voltage, charged particles remain in the discharge cells 41. Owing to these charged particles, even if the application of the writing voltage is stopped, a discharge can be maintained at a lower voltage (Vm) than the initial writing voltage (Vw) over a predetermined time period (normally, several micro seconds). The memory driving method operates the PDP by utilizing this phenomenon.
FIG. 22 shows a method for applying a voltage pulse in the case where the DC type PDP 500 shown in FIG. 20 is driven by the memory driving method.
Similarly to the refresh driving method, in the memory driving method, a writing voltage 54 of an amplitude Vw is selectively applied to predetermined discharge cells 43 by applying a negative pulse voltage 52 to the cathodes and a positive pulse voltage 53 to the anodes, thereby generating a discharge. In addition, after the application of the writing voltage 54, a maintaining pulse voltage 55 of an amplitude Vm is subsequently applied to the cathodes so as to prolong the discharge time period.
As described above, in the memory driving method, continuous light emission can be obtained by application of the maintaining pulse voltage 55 regardless of the number of cathode lines. Therefore, the luminance of the image to be displayed can be further enhanced as compared with the refresh driving method utilizing a light emission obtained only by application of the writing voltage. For example, the luminance of 150 cd/m.sup.2 or more, which is a sufficient value for television display, is accomplished.
The amplitude Vm of the maintaining pulse 66 is required to be set to a voltage Vpd or higher at which the discharge occurs (the discharge cells lighten) in the case where the writing voltage 54 is applied prior to the application of the maintaining voltage 55 and to a voltage Vxt or lower at which the discharge does not occur (the discharge cells do not lighten) in the case where the writing voltage 54 is not applied prior to the application of the maintaining voltage 55. The difference between these voltages (Vxt-Vpd) is called the memory margin and is generally about 20 V.
In the memory driving method, it is important to obtain a stable discharge voltage for realizing a stable operation of the DC type PDP. The discharge voltage is greatly affected by the cathode lines 43. Therefore, the cathode lines 43 are very important constituent components in the DC type PDP for reduction in power while the PDP is lightened, long-term stability of operation and reservation of the memory margin.
The cathode lines 43 may be formed of various materials such as metals and oxides. Conventionally, the cathode lines 43 are formed of Ni or an alloy thereof, mainly by screen printing.
Furthermore, a material having a low work function is deposited on the surface of the metal electrodes formed by screen printing in order to reduce the discharge voltage so as to reduce the power consumption of the DC type PDP. For example, Japanese Patent Publication Nos. 2-7136, 5-11381 and 5-11382 disclose such a structure.
FIGS. 23A and 23B schematically show the structure of cathode lines 59 disclosed in Japanese Patent Publication No. 2-7136. FIG. 23A is a cross-sectional view taken along a line 23A-23A' shown in FIG. 23B.
The cathode line 59 includes a base metal 56 and a porous adhesive layer 57 formed thereon. The base metal 56 is formed into a predetermined pattern (for example, in a stripe pattern in FIG. 23B) by screen printing. The porous adhesive layer 57 made of an oxide or a sulfide of alkaline earth metal elements, or a composite metal oxide of alkaline earth metal elements and aluminum is formed on the base metal 56 by a plasma spraying method in a predetermined pattern corresponding to the arrangement of the discharge cells. In FIG. 23B, the porous adhesive layer 57 is formed in a round shape. At least free alkaline earth metal elements 58 are present in a studded manner inside the pores of the porous adhesive layer 57.
In such a structure, an electrically insulating material or a material having a high melting point and a low work function is used as an electron-emitting material. By using the electron-emitting material, the discharge voltage is lowered, resulting in reduced power consumption. In the above-mentioned example, the oxide or the sulfide constituting the porous adhesive layer 57 is such a material of a low work function, which serves as the electron-emitting material.
In the case where the porous adhesive layer 57 made of these materials is formed by screen printing, in order that the porous adhesive layers 57 actually function as the cathode lines, it is necessary to perform a melting process and an activating process at a significantly high temperature after forming the porous adhesive layer 57 into a predetermined shape by screen printing, as a step for promoting the generation of free metal elements. On the other hand, in the case where the porous adhesive layer 57 is formed by the plasma spraying method, it is unnecessary to perform a high-temperature process since the plasma spraying step itself is performed at a high temperature. Thus, a cathode line of a low discharge voltage can be formed without applying a large heat load to the glass substrate after depositing the base metal 56 and the porous adhesive layer 57 on the glass substrate.
If the cathode lines are mainly formed by screen printing as described above, the DC type PDP can be fabricated using a relatively simple fabrication device. On the other hand, however, the formation of the cathode lines by screen printing has the following problems.
(1) Voltage drop due to the line resistance of the cathode lines:
Generally, in the DC type PDP, the cathode lines are sequentially scanned. In this process, if a number of discharge cells on one cathode line are simultaneously selected to be lightened, the current flowing through the discharge flows into the power source via the cathode line. Thus, a difference in voltage due to the line resistance of the cathode line is generated between an end on the power supply side and an end opposite thereto of the cathode line. As a result, as a distance from the power supply side becomes larger, the voltage which is actually applied to the discharge cells is lowered.
In the refresh driving method, this voltage difference appears as a luminance difference. Thus, the quality of the image to be displayed is degraded. In the case of the memory driving method, the memory margin is significantly deteriorated due to the voltage difference.
For example, a discharge current flowing into each discharge cell is about 60 .mu.A, when the electrode pitch is 200 .mu.m, the size of the cathode is 575 .mu.m (length).times.150 .mu.m (width), and He-Xe 10% is sealed, as the discharge gas, within the discharge cell under the pressure of 350 Torr. A sheet resistance of the cathode line having a thickness of 50 .mu.m formed of an aluminum print paste becomes about 40 m.OMEGA.. When the DC type PDP having about 900 anodes, which are necessary to an NTSC mode wide television, is constituted under the above conditions, the voltage difference between the power supply side end and the opposite side end of the cathode line is about 6 V. This implies that the memory margin is lowered by about 6 V at the opposite side end as compared with the power supply side end of the cathode line.
In this way, the line resistance bringing about a large voltage drop is one of the reasons for the lowered memory margin.
In the case where the cathode line is formed by screen printing, a metal paste for printing (glass frit) formed by mixing a binder such as a glass powder with a metal powder is generally used. Therefore, when the cathode line is formed by baking the paste which is screen printed into a predetermined pattern, the surfaces of metal particles are covered with the melted glass. As a result, the electric conductivity in the cathode line is lowered to about a fraction of that of metal, resulting in an increased line resistance. Therefore, in the cathode lines formed by screen printing, as the screen becomes larger, the line resistance increases because of conspicuous effects of the glass frit. This leads the degree of the voltage drop due to the current flowing through the cathode lines to be large. As a result, the quality of the image to be displayed is degraded, for example, the luminance in a length direction of the cathode line is lowered or some discharge cells are not lightened. In order to solve these problems, a driving voltage circuit is required to be large in scale. Consequently, it is difficult to reduce the fabrication cost or the size.
(2) Variation in a driving voltage during a lightening time period:
In the memory driving method for driving the PDP within a limited driving voltage range, it is necessary to limit the variation in the driving voltage during the lightening time period to a value as small as possible. However, in the case where, for example, a PDP having aluminum cathodes formed by screen printing is driven by the memory driving method, the driving voltage varies by about 15 V until the driving time period (aging time) reaches 30 thousand hours, for which a television for domestic use should be driving. As a result, the memory margin is remarkably lowered (by -15 V) during the lightening time period. As described above, the surfaces of the cathode lines formed by screen printing are generally covered with glass contained in the paste. As this glass coating is removed by the discharge during the driving, clean metal surfaces gradually appear, thereby varying the driving voltage.
Thus, in order to reduce the line resistance and the variation in the driving voltage during the lightening time period, the cathode lines of the DC type PDP are required to be formed in the state as close as possible to a pure metal.
Although it is also possible to form the cathode lines by vapor deposition, the vapor deposition method has problems in that a formable film is too thin to obtain a predetermined line resistance and the fabrication cost increases since a vacuum vapor deposition apparatus is needed.
In the plasma spraying method, a powdery cathode line material is blown into a jet stream in the high-temperature plasma state to melt the powdery material. The powdery material in the melted state is then adhered to the substrate at a high speed utilizing the energy of the jet stream. Therefore, the glass frit does not basically enter the cathode material, which was a problem in the screen printing method.
However, there is a problem in the process peculiar to the spraying method. In particular, in the case of the spraying utilizing powdery particles of a low specific weight or in the case where a fine pattern is formed over a large area, there arise many problems due to the principle of the spraying method. Therefore, the spraying method cannot be put into practical use as a method for forming the cathode line of the DC type PDP with high accuracy.
FIG. 24 schematically shows a method for forming the cathode line by plasma spraying. A glass substrate 60 serving as a front glass substrate of the PDP is directly put on a mounting table 65 made of metal and the like. Cathode line material particles 52 at a high temperature are collided at a high speed against the glass substrate 60 from a plasma spraying torch 61 provided above the glass substrate 60, thereby forming a thick film made of cathode line material on the surface of the glass substrate 60. The torch 61 or the substrate 60 is sequentially traversed in a direction indicated by an arrow 64 shown in FIG. 24 so as to perform spraying on the entire surface of the glass substrate 60. In this case, the actual cathode lines 63 are generally formed by using a lift-off method or the like from the thus formed thick film.
In the above conventional plasma spraying method, however, it is difficult to form the cathode lines 63 with a fine pitch and a fine width over the entire surface of the glass substrate 60 without disconnection. Although the glass substrate 60 on which the cathode lines 63 are formed is generally large in size, i.e., about 1 m.times.1 m, the thickness thereof is typically small, i.e., about 2 to 3 mm. When the plasma spraying is performed on such a thin and large glass substrate 60, a difference in temperature occurs between the region where the film deposition is currently being conducted by spraying and the remaining region. The glass substrate 60 may be broken by the thermal stress caused by such a temperature difference. Furthermore, since it is difficult to obtain a uniform thickness over the entire surface of the glass substrate 60, discharge characteristics may not be uniform. In particular, in the case where the narrow cathode lines 63 are to be formed over a large area using a metal of a small specific weight, it is difficult to attain appropriate characteristics of the cathode lines 63.
The problem described in the above point (1) similarly occurs in the case where the cathode lines are formed by using a spraying process. This is because, in the case where the cathode line is formed using the spraying process in a conventional method, a bus line (base metal) of the cathode line is formed by screen printing and the surface thereof is covered with the electron-emitting material by a spraying method.